In some instances Excel will try and ‘guess’ that you wanted a column of formula to continue to copy down as you make inputs. It is normally correct but sometimes you may want this functionality to not work.

You can switch it off by going to the options i.e. in 2007 click to windows logo on the top left, choose Excel Options, then Advanced, then Extend Data Range Formats and Formula’s as shown below.

How to switch off automatic format and formula extension